1. Field of the Invention
The invention relates to a method for Doppler LIDAR measurement of speeds by means of direct reception, and to an apparatus for carrying out the method.
LIDAR is short for “light detection and ranging” and is a method, which is closely related to radar (“radio-wave detection and ranging”), for range and speed measurement, as well as for remote measurement of atmospheric parameters. However, laser beams are used instead of radio waves, as in the case of radar.
One example of a Doppler LIDAR apparatus and of a method which can be carried out using it for measuring wind speeds is known from EP1756620 B1, US 20080117433 A1 or US 2006262324 A1.
In the case of direct reception Doppler LIDAR apparatuses, laser radiation is directed at the medium to be measured, and the radiation which is then reflected is received directly and is investigated for a Doppler shift in the laser wavelength, in order in this way to determine relative speeds.
Doppler LIDAR systems using direct reception technology are used, for example, for predictive measurement of turbulence, cross winds or vortex trains in front of an aircraft, in particular an airplane. In addition to a pure warning function, the measurement signals can in particular also be injected directly into the flight control system of the aircraft, in order, for example, to proactively regulate out gusts, cross winds or vortex train influences, that is to say before the aircraft reacts negatively to the external flow change, such that a constant smooth flight state is maintained, loads on the aircraft are reduced, and the safety in the aircraft and for the aircraft remains ensured.
One major problem in this case is the dynamics which occur in the back-scattering intensity. The intensity of the back-scattered signal can easily vary by three orders of magnitude because of the aerosol content and the air density. A major variation also occurs if the aircraft, for example an airplane, in which the LIDAR measurement system is located is flying through cloud banks, as a result of which the overall dynamics can easily amount to four to five orders of magnitude.
2. Background Information
Today's detectors have a very much narrower dynamic range. For example, CCD arrays which are used for so-called fringe imaging technology typically have a dynamic range of 10 to 12 bits. Based on a minimum intensity of 6 to 7 bits, the remaining dynamics are about one or two orders of magnitude.
Attenuation of the received signal, by way of example using electrooptical modulators, is highly complex, expensive and is difficult to achieve because of the normally large apertures for CCD chips.